This disclosure relates to the integration of catalytic and pyrolytic cracking units to produce olefins from a variety of feedstreams.
Olefins have long been desired as products from the petrochemical industry. Olefins such as ethylene, propylene, butenes, and pentenes are useful for preparing a wide variety of end products, including polyethylenes, polypropylenes, polyisobutylene and other polymers, alcohols, vinyl chloride monomer, acrylonitrile, methyl tertiary butyl ether and tertiary amyl methyl ether and other petrochemicals, and a variety of rubbers such as butyl rubber. A large number of processes, described in the literature, are directed to the production of olefins. In recent years, there has been an increasing demand for light olefinic gases while supplies of suitable feedstocks for producing such olefins have declined. Thus, the petrochemical industry is continuously looking for processes capable of providing improved flexibility in producing various olefins from hydrocarbon feedstocks.
This is especially true for the production of propylene. The largest source of petrochemical propylene on a worldwide basis is that produced as the primary byproduct of ethylene manufacture by thermal cracking. Ethylene plants charging liquid feedstocks typically produce about 10 to 20 weight percent propylene and provide about 70 percent of the propylene consumed by the petrochemical industry. Petroleum refining, predominantly from fluidized catalytic cracking (“FCC”), is by far the next largest supplier of worldwide propylene production, supplying about 30 percent of the petrochemical requirement. In the U.S., FCC's supply about one-half of the petrochemical propylene demand.
The demand for propylene is expected to more than double, primarily driven by the rapidly increasing market for polypropylene. Propylene demand by the petrochemical industry is projected to increase more rapidly than the demand for ethylene. Since ethylene plants produce more ethylene than propylene, and since many of the new ethylene plants in construction are based on ethane feed with no propylene co-produced, significant increases in propylene from FCC will be required to meet the increased demand.
U.S. Pat. No. 5,026,936 teaches a process for the preparation of propylene from C4 or higher feeds by a combination of cracking and metathesis wherein the higher hydrocarbon is cracked to form ethylene and propylene and at least a portion of the ethylene is metathesized to propylene. See also U.S. Pat. No. 5,026,935.
Processes for non-catalytically cracking and catalytically cracking hydrocarbon feedstocks are well known. Steam cracking in a furnace and contact with hot non-catalytic particulate solids are two well-known non-catalytic cracking processes. Exemplary processes are described in U.S. Pat. Nos. 3,407,789; 3,820,955; 4,499,055; and 4,814,067. Fluid catalytic cracking and deep catalytic cracking are two well-known catalytic cracking processes. U.S. Pat. Nos. 4,828,679; 3,647,682; 3,758,403; 4,814,067; 4,980,053; and 5,326,465 disclose exemplary processes.
There has been little activity to integrate catalytic and pyrolytic cracking processes with each other. U.S. Pat. No. 5,523,502 discloses a process design for olefin production incorporating an integrated deep catalytic cracking unit and a thermal cracking unit. Deep catalytic cracking is a process in which a preheated hydrocarbon feedstock is cracked over a heated solid acidic catalyst in a reactor at temperatures ranging from about 925° F. to about 1350° F. U.S. Pat. No. 6,033,555 discloses a process involving catalytic cracking of a hydrocarbon feedstock followed by thermal cracking.